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Date: Fri, 22 Jan 2016 13:33:41 +0100
From: Jesper Dangaard Brouer <brouer@...hat.com>
To: Eric Dumazet <eric.dumazet@...il.com>
Cc: Tom Herbert <tom@...bertland.com>,
Or Gerlitz <gerlitz.or@...il.com>,
David Miller <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>,
Linux Netdev List <netdev@...r.kernel.org>,
Alexander Duyck <alexander.duyck@...il.com>,
Alexei Starovoitov <alexei.starovoitov@...il.com>,
Daniel Borkmann <borkmann@...earbox.net>,
Marek Majkowski <marek@...udflare.com>,
Hannes Frederic Sowa <hannes@...essinduktion.org>,
Florian Westphal <fw@...len.de>,
Paolo Abeni <pabeni@...hat.com>,
John Fastabend <john.r.fastabend@...el.com>,
Amir Vadai <amirva@...il.com>, brouer@...hat.com
Subject: Re: Optimizing instruction-cache, more packets at each stage
On Thu, 21 Jan 2016 09:48:36 -0800
Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Thu, 2016-01-21 at 08:38 -0800, Tom Herbert wrote:
>
> > Sure, but the receive path is parallelized.
>
> This is true for multiqueue processing, assuming you can dedicate many
> cores to process RX.
>
> > Improving parallelism has
> > continuously shown to have much more impact than attempting to
> > optimize for cache misses. The primary goal is not to drive 100Gbps
> > with 64 packets from a single CPU. It is one benchmark of many we
> > should look at to measure efficiency of the data path, but I've yet to
> > see any real workload that requires that...
> >
> > Regardless of anything, we need to load packet headers into CPU cache
> > to do protocol processing. I'm not sure I see how trying to defer that
> > as long as possible helps except in cases where the packet is crossing
> > CPU cache boundaries and can eliminate cache misses completely (not
> > just move them around from one function to another).
>
> Note that some user space use multiple core (or hyper threads) to
> implement a pipeline, using a single RX queue.
>
> One thread can handle one stage (device RX drain) and prefetch data into
> shared L1/L2 (and/or shared L3 for pipelines with more than 2 threads)
>
> The second thread process packets with headers already in L1/L2
I agree. I've heard experiences where DPDK users use 2 core for RX, and
1 core for TX, and achieve 10G wirespeed (14Mpps) real IPv4 forwarding
with full Internet routing table look up.
One of the ideas behind my alf_queue, is that it can be used for
efficiently distributing object (pointers) between threads.
1. because it only transfers the pointers (not touching object), and
2. because it enqueue/dequeue multiple objects with a single locked cmpxchg.
Thus, lower in the message passing cost between threads.
> This way, the ~100 ns (or even more if you also consider skb
> allocations) penalty to bring packet headers do not hurt PPS.
I've studied the allocation cost in great detail, thus let me share my
numbers, 100 ns is too high:
Total cost of alloc+free for 256 byte objects (on CPU i7-4790K @ 4.00GHz).
The cycles count should be comparable with other CPUs, but that nanosec
measurement is affected by the very high clock freq of this CPU.
Kmem_cache fastpath "recycle" case:
SLUB => 44 cycles(tsc) 11.205 ns
SLAB => 96 cycles(tsc) 24.119 ns.
The problem is that real use-cases in the network stack, almost always
hit the slowpath in kmem_cache allocators.
Kmem_cache "slowpath" case:
SLUB => 117 cycles(tsc) 29.276 ns
SLAB => 101 cycles(tsc) 25.342 ns
I've addressed this "slowpath" problem in the SLUB and SLAB allocators,
by introducing a bulk API, which amortize the needed sync-mechanisms.
Kmem_cache using bulk API:
SLUB => 37 cycles(tsc) 9.280 ns
SLAB => 20 cycles(tsc) 5.035 ns
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
Author of http://www.iptv-analyzer.org
LinkedIn: http://www.linkedin.com/in/brouer
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